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containerd Plugins

containerd supports extending its functionality using most of its defined interfaces. This includes using a customized runtime, snapshotter, content store, and even adding gRPC interfaces.

Smart Client Model

containerd has a smart client architecture, meaning any functionality which is not required by the daemon is done by the client. This includes most high level interactions such as creating a container's specification, interacting with an image registry, or loading an image from tar. containerd's Go client gives a user access to many points of extensions from creating their own options on container creation to resolving image registry names.

See containerd's Go documentation

External Plugins

External plugins allow extending containerd's functionality using an officially released version of containerd without needing to recompile the daemon to add a plugin.

containerd allows extensions through two method:

  • via a binary available in containerd's PATH
  • by configuring containerd to proxy to another gRPC service

V2 Runtimes

The runtime v2 interface allows resolving runtimes to binaries on the system. These binaries are used to start the shim process for containerd and allows containerd to manage those containers using the runtime shim api returned by the binary.

See runtime v2 documentation

Proxy Plugins

A proxy plugin is configured using containerd's config file and will be loaded alongside the internal plugins when containerd is started. These plugins are connected to containerd using a local socket serving one of containerd's gRPC API services. Each plugin is configured with a type and name just as internal plugins are.

Configuration

Update the containerd config file, which by default is at /etc/containerd/config.toml. Add a [proxy_plugins] section along with a section for your given plugin [proxy_plugins.myplugin]. The address must refer to a local socket file which the containerd process has access to. The currently supported types are snapshot and content.

[proxy_plugins]
  [proxy_plugins.customsnapshot]
    type = "snapshot"
    address = "/var/run/mysnapshotter.sock"

Implementation

Implementing a proxy plugin is as easy as implementing the gRPC API for a service. For implementing a proxy plugin in Go, look at the go doc for content store service and snapshotter service.

The following example creates a snapshot plugin binary which can be used with any implementation of containerd's Snapshotter interface

package main

import (
	"fmt"
	"net"
	"os"

	"google.golang.org/grpc"

	snapshotsapi "github.com/containerd/containerd/api/services/snapshots/v1"
	"github.com/containerd/containerd/contrib/snapshotservice"
	"github.com/containerd/containerd/snapshots/native"
)

func main() {
	// Provide a unix address to listen to, this will be the `address`
	// in the `proxy_plugin` configuration.
	// The root will be used to store the snapshots.
	if len(os.Args) < 3 {
		fmt.Printf("invalid args: usage: %s <unix addr> <root>\n", os.Args[0])
		os.Exit(1)
	}

	// Create a gRPC server
	rpc := grpc.NewServer()

	// Configure your custom snapshotter, this example uses the native
	// snapshotter and a root directory. Your custom snapshotter will be
	// much more useful than using a snapshotter which is already included.
	// https://godoc.org/github.com/containerd/containerd/snapshots#Snapshotter
	sn, err := native.NewSnapshotter(os.Args[2])
	if err != nil {
		fmt.Printf("error: %v\n", err)
		os.Exit(1)
	}

	// Convert the snapshotter to a gRPC service,
	// example in github.com/containerd/containerd/contrib/snapshotservice
	service := snapshotservice.FromSnapshotter(sn)

	// Register the service with the gRPC server
	snapshotsapi.RegisterSnapshotsServer(rpc, service)

	// Listen and serve
	l, err := net.Listen("unix", os.Args[1])
	if err != nil {
		fmt.Printf("error: %v\n", err)
		os.Exit(1)
	}
	if err := rpc.Serve(l); err != nil {
		fmt.Printf("error: %v\n", err)
		os.Exit(1)
	}
}

Using the previous configuration and example, you could run a snapshot plugin with

# Start plugin in one terminal
$ go run ./main.go /var/run/mysnapshotter.sock /tmp/snapshots

# Use ctr in another
$ CONTAINERD_SNAPSHOTTER=customsnapshot ctr images pull docker.io/library/alpine:latest
$ tree -L 3 /tmp/snapshots
/tmp/snapshots
|-- metadata.db
`-- snapshots
    `-- 1
        |-- bin
        |-- dev
        |-- etc
        |-- home
        |-- lib
        |-- media
        |-- mnt
        |-- proc
        |-- root
        |-- run
        |-- sbin
        |-- srv
        |-- sys
        |-- tmp
        |-- usr
        `-- var

18 directories, 1 file

Built-in Plugins

containerd uses plugins internally to ensure that internal implementations are decoupled, stable, and treated equally with external plugins. To see all the plugins containerd has, use ctr plugins ls

$ ctr plugins ls
TYPE                            ID                    PLATFORMS      STATUS
io.containerd.content.v1        content               -              ok
io.containerd.snapshotter.v1    btrfs                 linux/amd64    ok
io.containerd.snapshotter.v1    aufs                  linux/amd64    error
io.containerd.snapshotter.v1    native                linux/amd64    ok
io.containerd.snapshotter.v1    overlayfs             linux/amd64    ok
io.containerd.snapshotter.v1    zfs                   linux/amd64    error
io.containerd.metadata.v1       bolt                  -              ok
io.containerd.differ.v1         walking               linux/amd64    ok
io.containerd.gc.v1             scheduler             -              ok
io.containerd.service.v1        containers-service    -              ok
io.containerd.service.v1        content-service       -              ok
io.containerd.service.v1        diff-service          -              ok
io.containerd.service.v1        images-service        -              ok
io.containerd.service.v1        leases-service        -              ok
io.containerd.service.v1        namespaces-service    -              ok
io.containerd.service.v1        snapshots-service     -              ok
io.containerd.runtime.v1        linux                 linux/amd64    ok
io.containerd.runtime.v2        task                  linux/amd64    ok
io.containerd.monitor.v1        cgroups               linux/amd64    ok
io.containerd.service.v1        tasks-service         -              ok
io.containerd.internal.v1       restart               -              ok
io.containerd.grpc.v1           containers            -              ok
io.containerd.grpc.v1           content               -              ok
io.containerd.grpc.v1           diff                  -              ok
io.containerd.grpc.v1           events                -              ok
io.containerd.grpc.v1           healthcheck           -              ok
io.containerd.grpc.v1           images                -              ok
io.containerd.grpc.v1           leases                -              ok
io.containerd.grpc.v1           namespaces            -              ok
io.containerd.grpc.v1           snapshots             -              ok
io.containerd.grpc.v1           tasks                 -              ok
io.containerd.grpc.v1           version               -              ok
io.containerd.grpc.v1           cri                   linux/amd64    ok

From the output all the plugins can be seen as well those which did not successfully load. In this case aufs and zfs are expected not to load since they are not support on the machine. The logs will show why it failed, but you can also get more details using the -d option.

$ ctr plugins ls -d id==aufs id==zfs
Type:          io.containerd.snapshotter.v1
ID:            aufs
Platforms:     linux/amd64
Exports:
               root      /var/lib/containerd/io.containerd.snapshotter.v1.aufs
Error:
               Code:        Unknown
               Message:     modprobe aufs failed: "modprobe: FATAL: Module aufs not found in directory /lib/modules/4.17.2-1-ARCH\n": exit status 1

Type:          io.containerd.snapshotter.v1
ID:            zfs
Platforms:     linux/amd64
Exports:
               root      /var/lib/containerd/io.containerd.snapshotter.v1.zfs
Error:
               Code:        Unknown
               Message:     path /var/lib/containerd/io.containerd.snapshotter.v1.zfs must be a zfs filesystem to be used with the zfs snapshotter

The error message which the plugin returned explains why the plugin was unable to load.

Configuration

Plugins are configured using the [plugins] section of containerd's config. Every plugin can have its own section using the pattern [plugins.<plugin id>].

example configuration

[plugins]
  [plugins.cgroups]
    no_prometheus = false
  [plugins.cri]
    stream_server_address = ""
    stream_server_port = "10010"
    enable_selinux = false
    sandbox_image = "k8s.gcr.io/pause:3.1"
    stats_collect_period = 10
    systemd_cgroup = false
    [plugins.cri.containerd]
      snapshotter = "overlayfs"
      [plugins.cri.containerd.default_runtime]
        runtime_type = "io.containerd.runtime.v1.linux"
        runtime_engine = ""
        runtime_root = ""
      [plugins.cri.containerd.untrusted_workload_runtime]
        runtime_type = ""
        runtime_engine = ""
        runtime_root = ""
    [plugins.cri.cni]
      bin_dir = "/opt/cni/bin"
      conf_dir = "/etc/cni/net.d"
    [plugins.cri.registry]
      [plugins.cri.registry.mirrors]
        [plugins.cri.registry.mirrors."docker.io"]
          endpoint = ["https://registry-1.docker.io"]
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